Use the magnetic force


An ammeter is a device that detects an electric current. With some readily available materials, you can build your own ammeter and use it to measure current produced by batteries including homemade batteries and generators.




Cut a 4 cm (1.5 inch) length from the tube.
Wind a coil of at least 100 turns of magnet wire around the section of cardboard tube, leave at least 10 cm of wire free at each end of the coil. Use masking tape or hot melt glue to hold the coil in place on the tube.

Hot melt glue, or staple, the paper tube to the base.

Sandpaper the ends of the magnetic wire to remove the insulating layer of enamel. The ends of the wire should appear as shiny copper.

Take one free end of the wire from the coil make it into a loop and twist the loop several times. Fold the aluminum foil around the looped bare ends of the wire until it makes a rectangle, several layers thick and about 1 cm x 2.5 cm. Use a pushpin or staple the aluminum foil pad to the cardboard. Do the same with the other free end of the wire. Mark a + sign near one end of the wire, it does not matter which end.

Hang the magnet from a string in the center of the coil: ,make a string sandwich, hold the string between the two disk magnets. Make a slit in the center of the top of the tube, run the slit up to the edge of the coil. Make another slit on the other side of the coil. Hang the magnet by slipping its thread through the first slit then slip the thread through the second one. The friction of the thread going through two slits will hold the magnet in place. Adjust the thread so that the magnet hangs in the middle of the coil.

To do and notice

The hanging magnet aligns itself with the earth's magnetic field, just like a compass needle. (see the activity titled, "Where's North?")
To use the ammeter, turn the coil so that the coil is lined up north and south. When you look in the end of the tube you should see the edge of the magnet.

End view of the ammeter with the magnets aligned with the Earth's magnetic field.

Attach the battery to the leads. Attach the + side of the battery to the + lead of the meter. The hanging magnet should turn so that one face is toward the aluminum pads. Attach a piece of tape to that face and print + on the tape.
Reverse the connections to the battery the magnet should rotate the opposite direction.

Your ammeter is very sensitive. The magnets in ours deflect 45 degrees when 10 milliamps flow through them. To measure the sensitivity of your meter connect a calibrated ammeter in series with a battery and a variable resistor. Change the resistance until the ammeter magnet turns 45 degrees read the current on the calibrated ammeter

You can use your ammeter to measure the current produced by your own lemon batteries or home-made generators.

What's Going On?

The electric current running through the coil of wire produces a magnetic field. The coil has a north pole end and a south pole end. The magnetic field of the electromagnet pushes on the the hanging magnet, making it twist. The hanging magnet is held in its no-current position by the earth’s magnetic field. The stronger the current in your coil, the stronger the magnetic field produced by the coil and the more the magnet twists.

When the magnetic field of the electromagnet equals the magnetic field of the earth the magnets will hang at a 45 degree angle from a line down the axis of the tube.

Going Further

Measure the current through a lamp from a AA battery, try a 2 volt christmas tree light from a string of 50 lights.

Measure the current through a motor made by a AA battery.

Measure the current from a lemon battery.

Measure the current from a solar cell.

Alternative construction

Build the same as above but use PVC tubes 1 inch in diamtere instead of cardboard tubes.


with a compass

Wrap a coil of 100 turns of wire around the compass. Wind the coil so that it passes over the north and south end of the compass, and yet still allows you to see the compass needle.

Making the ammeter quantitative

Replace the earth with the largest magnet you can find.

A large magnet can be used to calibrate the ammeter.

Use a callibrated ammeter in series with your ammeter and move the large magnet until the central disk magnets make a 45 degree angle with the axis of the tube. Then make a mark on the base to indicate the position of the large magnet.

Return to Day 15

Scientific Explorations with Paul Doherty

© 1999

30 May 2000